1. Field of Invention
This invention relates generally to fabrics having cholesteric liquid crystals integrated therewith to impart temperature-sensitive chromatic properties thereto, and more particularly to articles of apparel which include such fabrics and which exhibit a color pattern that undergoes changes in the course of wear.
2. Status of Prior Art
A liquid crystal is an organic compound having properties that appear to be simultaneously fluidic and crystalline. The molecules of the crystal are actually at an intermediate thermodynamic state of matter, lying between the liquid and crystalline phases.
A cholesteric liquid crystal is formed in layers, each containing long molecules with their long axes parallel to the plane of the layer. The molecules are substantially flat but include a side chain of methyl groups that project upwardly from the plane of each molecule. The displacement necessary to accommodate this side chain causes rotation of the direction of the molecules from one layer to the next, and this gives rise to a helical structure having unique optical properties.
Cholesteric liquid crystals undergo chromatic changes when subjected to variations in temperature. In the early 1960's, James Fergason discovered that the iridescent spectral response of cholesteric liquid crystals could be exploited to measure thermal changes. (See J. Fergason, "Liquid Crystals," Scientific American 210, 77 (1967 ), and H. W. Gibson, "Cholesteric Liquid Crystals"--Liquid Crystals, The Fourth State of Matter (1979), Ed. F. D. Seeva, Marcel Decker, Inc., Chapter 3, 99).
Liquid crystal components, such as cholesteryl benzoate, cholesteryl chloride and cholesteryl oleate are commercially available, one source being the Pressure Chemical Company of Pittsburgh, Pa. This company offers a series of liquid crystal mixtures in semi-solid or in solution form to cover a series of temperature ranges. Each mixture is calibrated so that each color change occurs at a specific temperature. Thus, one can with one mixture produce a red color at, say, 110.degree. F., the same temperature producing yellow with another mixture. Hence, one can predetermine the relationship of temperature to crystal color.
The semi-solid liquid crystal mixture can be heated to liquifaction and a thin film thereof spread on a substrate. When a thin film of the liquid crystal mixture is applied to a test surface and the surface is then heated or cooled, a thermograph of vivid colors is produced. While the film at ambient temperature is colorless, heating causes the film to become red, then yellow, green, blue, and violet, the changing colors being indicative of a progressively increasing temperature level. If, however, the test surface is cooled, the color sequence then takes place in reverse order. Thus, in this application, the liquid crystal film may be used for the measurement of surface temperature and also for non-destructive testing. Since liquid crystals selectively scatter rather than absorb light, a dark background is required for maximum color visibility.